In the video where we first
introduced the Milankovitch cycles where we talk
about the precession and how the tilt of the Earth,
the obliquity, can also change, I get this comment here on the
YouTube channel from Vicksoma and he or she says, if I
understood this correctly, precession changes the
time of the seasons over long periods of time,
and obliquity changes the strengths of the season
over long periods of time. And so this is a good comment. So the first comment
isn't exactly true, and that's what I'm going
to focus on in this video. He says or she says
precession changes the time of the seasons
over long periods of time. This is kind of true, but
I don't think in the sense that Vicksoma is
referring to it. The second part is roughly true. Obliquity changes the
strengths of the season over long periods of time. If you are more tilted towards
the sun in the extreme, then, yes, you will
have a bigger disparity than when you are
less tilted, when you are tilted
away from the sun. Or I guess you'd
say, if you are more tilted to or away from
the sun, the disparity between summer and
winter will be greater than if you are less tilted. So the second statement is
true, although you always have to be careful with things
as complicated as the climate because it can really depend
from parts of the global to parts on the globe
depending on what are all of the other
factors that play into it. So you would want to run some
type of simulation or something like that, but the second
part is roughly true. But I want to focus
on the first part because I think
it'll really give us a better understanding
of what precession is. And so this last
statement Vicksoma makes is actually not true. So he says-- I'm assuming
it's a he, maybe it's a she-- so in a
several thousand years, in several thousand years-- not
a-- in several thousand years, if we still use the same
calendar system, summer and winter will happen
in different months and they will be more
mild or more harsh. And what we're going to see
is that the second statement is not true because our
calendar is actually based on when we are most tilted
away from or towards the sun. So our calendar
is actually based on-- is actually
to some degree you could say takes
precession into account, and what actually does change
according to our calendar is when we are closest or
furthest away from the sun-- the perihelion or aphelion. And we're going to think
about that in this video. So let me draw the sun here. Let me draw the sun
right over here. And let me draw Earth's
orbit around the sun. And I'm going to draw it
with some eccentricity. And just so you know what
I'm talking about when I say eccentricity, a
circle has no eccentricity. An ellipse, this
ellipse right here has more eccentricity
than this circle, which has no eccentricity. And an even more eccentric
ellipse would look like this. So you can really think
about eccentricity as a measure of how
far you are away from being perfectly circular. So Earth's orbit around the sun
is pretty close to circular, but it has some eccentricity. It is slightly
elliptical, and I'm going to exaggerate it a bit in
this drawing right over here. So let's say that this is the
closest point that Earth gets to the sun so that
is the perihelion. And let's say this is
the furthest point. And obviously, it's not
this big of a difference. It's actually only a 3%
difference right now, but we'll also learn
that that's changing. But it never becomes
this dramatic, but this will help
us visualize it. So Earth's orbit might
look something like this. Earth's orbit--
let me make it, I can do a better job than
that-- Earth's orbit might look something like this. Obviously, I'm exaggerating,
though, the eccentricity here. So let's say that's
Earth's orbit. This is the point where we are
closest, so that's perihelion. And this is when
we are furthest. This is aphelion. And we saw in the first
video when we discussed this that right now this perihelion
is occurring in January, and this will change over time
as we'll see in this video. So January right now,
and aphelion right now occurs in July. Now, the time where we are
most tilted towards the sun is not at the
perihelion right now. It actually occurs a few
weeks before the perihelion. So when we are most
tilted to the sun, this is our winter solstice. And this is when
we are-- actually, in the case of the
Northern Hemisphere, this is when we are most
tilted away from the sun, I should say. So if we were to
draw our tilt here, if I were to come
out of the North Pole it would look like that. And this right over here,
depending on the year and where you are in your
time zone and everything, this is usually
December 21st or 22nd. I'll just go with
December 22nd for now. And when the Northern Hemisphere
is most tilted towards the sun occurs on June 20th or 21st,
so roughly six months later. Or really exactly
six months later. It's just that all the months
have different numbers of days and you have leap year sometimes
with February sometimes having 28 or 29 days, but if
you go half a year away then you are at-- in the case
of the Northern Hemisphere-- the summer solstice,
and this is when we are most tilted
towards the sun. And once again, it occurs
right now a few weeks before the aphelion, before we
are furthest away from the sun. Now, I want to zoom in a right
over here on December 22nd. So right over here,
let me zoom in. So let's say that
this is the Earth. I'm zooming in on this
circle right over here so let me box it to
show this is the one I am focused on right over here. And let me draw the
axis of rotation. And we know that that angle
versus the vertical that you could call the tilt
or the obliquity and we know this is
23.4 degrees relative to the vertical-- relative
to perpendicular compared to the plane of
our orbit, I guess. So if our orbital axis
was straight up and down, it would look
something like this. It's not, it tilts, and
this angle right over here is 23.4 degrees. And when I say
straight up and down, I'm saying relative to
the plane of Earth's orbit around the sun. So this right here
is the obliquity. And as Vicksoma mentioned,
this does change, it goes between 22 degrees
and 24 and 1/2 degrees over very long periods of
time, but it does-- oh, I think it's 41,000 years if
I remember that correctly-- so it will affect on
some level the severity. So this tilt is going between
that where actually it's reducing right now,
and it will get to a minimum in a
few thousand years. So it will get to some
minimum and then eventually it will get back to
some maximum tilt so it goes back and
forth between those two over the course of several
tens of thousands of years. But anyway, this is as it
is zoomed in right now. And as we mentioned,
precession you can view it as if this arrow right
here actually existed, it would trace out a
circle and is tracing out of this circle over a
huge period of time, over 26,000 years. So let me make
everything clear here. Right now, I'm just going
to assume Earth is rotating and that the orbital direction
is in that direction. I'm going to assume-- let me
make that a little bit more curved-- this is the
rotation of the Earth, it is in this direction
right over here. And what we learned about
precession-- and actually, to be particular,
it's axial precession. There's multiple types of
precession we will talk about. If someone says just
precession they're usually referring
to axial precession. It's this idea that over 26,000
years, the tip of this arrow, or you could even imagine
the poles themselves, will trace out a circle. If you look at the
same point in our orbit at any period in time, the
circle that is tracing out is going to be going in this
direction right over here. So if we wait
1,800 years-- and I want to make sure I get
this right because it's important to see what
happens to our calendar-- if we wait 1,800
years this arrow, it will still have a
tilt of 23.4 degrees, but instead of pointing
in this direction, it might be pointing
in this direction. Or in fact, it will be
pointing in this direction. I'm obviously not
drawing it that exact. And then the bottom of the
arrow will come out over here. So if you think about that,
if you wait 1,800 years, and once again, the
tilt hasn't changed or it's changed a little bit,
but what the precession has done, tracing out this circle
has changed the direction of this arrow, it
changed the direction of our axis of rotation. And if you wait 1,800 years when
will the Northern Hemisphere be pointed most away from the sun. Well, now, it won't
be pointed most away from the sun at this point
in space relative to the sun anymore because now
its axis of rotation looks something like this. So now if we wait-- or I
should say in 1,800 years, it will be most pointed away,
or the Northern Hemisphere will be most pointed away from
the sun, about a month earlier. So about a month earlier. It'll be most pointed away from
the sun about a month earlier. So this is when it will be
most pointed away from the sun. To today's time,
we would say, no, that's still not the
most pointed away, but since we have
this precession, since the direction of the
tilt I guess we could say, or the direction of our
rotational axis is changing, we are now at a different
point in our orbit where we are most pointed
away from the sun. So this is 1,800 years
later, approximately. So now based on
this, and I think this is what Vicksoma might have
been hinting at, you say, look, OK, it's earlier in our orbit. Wouldn't this now be November? And the answer is no. It will still be December 22nd. This will still be December 21st
or 22nd Depending on the year. Still be the same
date, and that's because our calendar is based
on when we are most tilted away or when we are most
tilted towards the sun. So by definition, this is
when we are most tilted away so this will be the
winter solstice. So what happens
is every year-- so the way I drew it
right over here and, actually, this
perihelion actually changes over time as well. There's a precession of
the perihelion as well, but I'm not going to
go into that right now. So if you fast
forward 1,800 years, all that's going to
happen is that what we consider by
our calendar to be December 22nd in an
absolute point in our orbit we'll be earlier in our
orbit, but we're still going to call it December 22nd. And so the perihelion is going
to be further away from that December 22nd, it's actually
going to be a month further away, so the perihelion
1,800 years from now won't be in January,
it will be in February. So the real takeaway
here is that our calendar is based not on the exact point
in space relative to the sun. Our calendar is based on the
maximum tilt towards or away from the sun, and
that, as we see, that is slightly
changing in terms of where it occurs in the
absolute point in space. I think it's changing by
roughly 20 minutes a year. So every year, the perihelion
is getting 20 minutes later. If we wanted to
use the perihelion, if we wanted to use the exact
point in space as our calendar, our year would actually
be about that-- I don't know what it is-- roughly
20 or 25 minutes longer, but it actually makes a
little much more sense to think about it from
the tilt because that's what dictates the seasons,
and that's actually what's most
observable from Earth, where the sun is in the sky.